Electron heat diffusivity in the sawtoothing tokamak core

The electron heat diffusivity has been measured in the sawtoothing core of the TEXT-U tokamak plasma during electron cyclotron heating (ECH). ECH creates a non-steady-state electron temperature (T-e) gradient on a time scale faster than a single sawtooth ramp, whose relaxation is measured with two ECE systems. The measured diffusivity is considerably smaller than that measured in the steady state during the ohmic phase of the discharge outside the sawtooth inversion radius. In these conditions, high-resolution ECE has also shown the presence of small scale, non-symmetric structures characterized by very large temperature gradients. These spatially localized, temporally cyclical structures, previously described as plasma filamentation, have been identified as plasma magnetic islands. If these small-scale gradients are taken into account then the reported measurements imply an electron heat diffusivity which approaches the neoclassical prediction.